In many powered tools a clutch is interposed between a driving motor and the actual tool and which work is to be done. This is almost invariably the case where the motor is an internal combustion engine. At engine idling speed the clutch is disengaged and the tool is inactive. When the driven part of the tool, e.g. a chain saw, a brushcutter or a lawn edger, etc., is to be activated the engine revolutions are increased and this increased the centrifugal force acting on a movable clutch member and clutch engagement results.
Because of the small space usually available for such clutches there has been a tendency to design intricate precision clutch mechanisms for such machines and tools. The clutch mechanisms include a driving assembly having a number of movable shoes and a driven part engagable by the shoes. Commonly, the driving assembly includes a clutch shoe support called a clutch hub for fixing to a drive shaft and the hub is provided with radially arranged clutch shoe guide means. Clutch shoes supported by the guide means are designed to move along the guide means under the influence of centrifugal force against the action of return springs acting on the clutch shoes. The clutch shoes are urged by the centrifugal force into driving contact with the internal surface of a drum, which constitutes the clutch mechanism driven part, and which houses the clutch shoes. The drum is drivingly connected to the tool to be operated.
Centrifugal clutches of common form depend on the centrifugal force acting on the clutch shoe to overcome the return spring tension, and for the clutch shoe to "lock" onto the clutch drum by friction grip. It has always been standard practice typically for the abovementioned tools to make solid, and therefore heavy, wide surface (usually 12-14 mm width) clutch shoes to achieve a good friction grip, which results from the greater centrifugal force available by using heavier shoes.
Typically, such clutch hub and clutch shoes are made by a powder metallurgy (PM) process as a sintered product. The PM parts individually made in special multiple action presses with powdered metal being compressed into a "biscuit like" consistency. The items are then baked in an oven to fuse the powder into a rigid state. This process from a manufacturing point of view is slow and capital intensive for both the tooling and PM presses. The items are thus expensive to manufacture.
Another clutch driving arrangement is made up of a driven clutch hub to which is pivotally connected multiple (usually two) clutch shoes, with the pivot point at or near one end of each clutch shoe. When the clutch shoes, with the pivot point at or near one end of each clutch shoe. When the clutch shoes swing out, the arcuate shoes engage the surface of the clutch drum. This occurs when the required RPM is sufficient to overcome the restraining spring tension of the restraining spring. Usually one spring is used connecting the two outer ends of the two shoes. The hub and clutch shoes for this clutch arrangement can be made by the PM process.
Another manufacturing process involves manufacturing a number of components stamped (blanked) from steel sheet metal, usually about 2 mm thick, which are then stacked and riveted together, usually about 6 or 7 pieces (laminates) each approximately 2 mm in thickness are required to achieve the required 12 mm to 14 mm surface width, thickness and weight for clutch shoes manufactured in this manner. If the clutch shoe is intricate in shape, for example having small holes for engagement by the ends of springs and with small radii at various corners, then to increase stamping tool life, thinner sheet metal would be used. It follows that more laminates would then be required to achieve the required surface width, show thickness and weight.
Another method of manufacturing is by die casting of aluminium or zinc, to form either or both the clutch hub and/or the clutch shoe. This is a lower cost method of production than the PM process but these cast clutch shoes suffer the problem of fast frictional wear and therefore is not practical in moderate to high use situations. To reduce the substantial wear on cast clutch shoes, linings (of material similar to automotive brake linings) are glued to the outer curved area of the cast clutch shoe which makes contact with the clutch drum. This is an expensive added step in the manufacturing process and negates the cost savings of die casting.
The purpose of the clutch mechanism in the unit driven by an internal combustion engine is two fold, firstly to allow the engine to idle over with no clutch engagement and secondly to prevent damage to the power unit in the case of overload, possibly as a result of a tool driven by the clutch becoming jammed. In the latter case when the load on the tool as a result of the jamming exceeds the clutch grip the clutch will slip. Where the power unit is an electric motor the clutch is provided as an overload avoidance means to prevent damage to the electric motor when the driven tool jams.
One object of this invention is to provide a simple wide surface construction driving assembly for a centrifugal clutch which is simpler and less expensive to make than the driving assemblies now commonly in use and generally as described above. The economy arises out of the use of parts which are few in number, involve a much lower material cost and are more readily and less expensively made than those used in the described prior art driving assemblies.
Another object of the invention is to provide a very light weight clutch mechanism, thereby reducing the weight of hand held tools.
A third object of the invention is to allow, there the power is provided in an on/off sequence (as with an electric motor as a power source), the possibility of elimination of return springs, but an arrangement where the clutch mechanism provides, smooth unabrupted power transmission and still provides overload protection through the clutch shoes.